Most of our knowledge of visual developmental mechanisms during the postnatal "critical period" is inferred from studies of adult cats raised with lid-suture. Few studies have actually been made during the critical period, and that is the purpose of present application. Normal, monocularly deprived (MD) and binocularly deprived (BD) kittens will be studied at various postnatal ages (20-90 days) to chart the time course and dynamics of neuroplastic development in the lateral geniculate nucleus (LGN) and striate cortex. Physiologically, receptive field (RF) methods will be used to study the development of LGN X- and Y-cells plus cortical simple and complex cells. Anatomically, LGN cell measurements as well as an assessment of geniculocortical projections by the horseradish peroxidase technique will be performed. The relative time course of abnormalities described for the adult will be determined, and comparisons of the relative effects of deprivation upon binocular and monocular segments as well as MD and BD cats will be emphasized. We anticipate that these data will provide important insights into the mechanisms whereby the environment interacts with the developing, neuroplastic visual system.